The long-range goal of this research is to understand the process by which simian virus 40 (SV40) acts to transform cells growing in tissue culture and to cause tumors in susceptible animals. We propose to examine the roles that two SV40-encoded proteins, t and T, may play in transformation and oncogenicity. For our overall research plan, we propose to isolate and characterize SV40 mutants that should help us determine the functions of t and T, particularly with respect to the initiation and the maintenance of transformation. We have isolated mutants of the type t-Tts, t-T+, and t+T- and are screening for ttsT+ mutants. We have characterized these mutants and are isolating cells transformed by them to study the roles of t and T in the maintenance of transformation. To facilitate our isolation and characterization of SV40 deletion mutants, we have also worked to increase the infectivity of SV40 DNA for mammalian cells with respect to the percentage of cells in the populaton that undergo successful infection. We have increased the percentage of monkey cells that become positive for T-antigen after exposure to SV40 DNA to greater than 20%, and we are attempting to make infection of cells with DNA as efficient as infection with SV40 virions.